How to Model a Mushroom shape?
If a Screw Modifier isn't already enough, simple Polygon modelling will be surely.


add a 5 (or more..) sided circle
extrude up and scale to give hints of the shape
then stick a subsurf modifier on it
(optional) then use a lattice or a curve modifier for deformation. Deform to taste.




The lattice:



The Curve Modifier: